A tectonic plate or tectonic plate is a piece of the Earth's surface that moves as a whole. There are seven major tectonic plates on Earth, and several smaller ones. These plates shift at a very slow rate, at most a few centimeters per year. The mechanism behind this is known as plate tectonics. The lithosphere - the planet's brittle outer layer - lies on the much fainter asthenosphere, which can flow slowly in a plastic way. Convection currents keep the asthenosphere in constant motion. The tectonic plates that make up the lithosphere are pushed by that movement.
Tectonic plates don't necessarily stop at the edge of a continent or the continental shelf. In some cases, the adjacent ocean floor belongs to the same plate. Some plates therefore consist of both continental and oceanic parts. Other plates consist entirely of either type of lithosphere. Incidentally, there is also an "intermediate" lithosphere, which is in between the two types in composition.
The junction of two plates can be a convergent, divergent or transform plate boundary. At convergent plate boundaries, two plates move toward each other - usually one plate moves below the other. Divergent plate boundaries are places where two plates move apart, leading to extension or oceanic spreading. Transform plate boundaries are boundaries where the plates slide past each other, a situation that occurs along the San Andreas Fault between the Juan de Fuca Plate and the North American Plate.
The edges of continents (continental margins) can be active or passive. An active margin is also a plate boundary, a passive margin is not a plate boundary but merely a transition from continental to oceanic lithosphere in the same plate.
Overview of tectonic plates
Although plate boundaries are often distinct, this is not the case everywhere. The geological structures are in many places more complex than can be represented by a single line on a map. Therefore, small differences exist in the models for the division of the Earth's surface into plates. Most models define seven major and about eight minor tectonic plates.
North American Plate
South American Plate
Juan de Fucaplaat
In addition to the large and smaller plates, there is also a (large) number of microplates. These usually lie along a plate boundary between two larger plates, and have started to move loosely due to the friction in such an area. In some cases it is difficult to determine whether one is dealing with a separate plate or a tectonic unit such as a large thrust fault (a wrapper). Often these microplates can be identified by the deeper structures in the lower crust, using seismological surveys and tomography. Examples of microplates are:
Anatolian Plate (Turkey)
Apulian Plate (Italy)
Explorer Plate (Canada)
Galapagos Plate (Ecuador)
Iberian Plate (Spain and Portugal; the Iberian Peninsula)
Rivera Plate (Mexico)
Tonga plate (Tonga)